Impairment of DNA damage repair following irradiation. (A) U251 cells were pretreated with the indicated inhibitors plus TMZ before assessment of γH2AX foci formation. Mock-treated control cells were analyzed 6 h after irradiation with 6 Gy. Pretreatment of U251 cells with the dual inhibitor PI103, the HSP90 inhibitor 17-DMAG, or the HDAC inhibitor LBH589 plus TMZ caused marked prolongation of radiation-induced γH2AX foci formation 6 h after 6Gy irradiation. (B) Pretreatment of U251 cells with TMZ combined with PI103, 17-DMAG, or LBH589 attenuated p-DNA-PK expression.